Potassium disposable optical sensor based on transflectance and cromaticity measurements

This paper presents the study of a disposable optical sensor to determine potassium by an ion-exchange mechanism based on the conventional ionophore dibenzo-18-crown-6. The normalised signal used as analytical parameter is obtained from measurements in transflectance mode and the results are compared with those obtained by transmission mode. The sensor responds linearly in activities up to 100 mM with a detection limit of 19 mM and a precision around 7%. It is possible to linearise the sigmoidal calibration curve by means of a decimal logistic transformation applied to transflectance data, in this way increasing the linear dynamic range with a detection limit of 8.4 mM. The potential utility of the disposable sensor was checked in the determination of potassium in different types of waters, validating the results against a reference procedure. The viability of the semi-quantitative determination of potassium using this disposable sensor was studied using a colour-difference CIELAB-base formula, designed to give quantitative values of the perceived colour difference between a pair of coloured samples under specific experimental conditions, obtaining seven intervals of discriminable concentrations in the colour chart between 10−7 and 10−1 M.